Organizations, including businesses, governments and educational institutions, increasingly rely on computer networks to share and exchange information. A computer network typically comprises a plurality of interconnected entities. An entity may consist of any device, such as a host or server, that sources (i.e., transmits) and/or receives messages. A common type of computer network is a local area network (“LAN”) which typically refers to a privately owned network within a single building or campus. In many instances, several LANs may be interconnected by point-to-point links, microwave transceivers, satellite hook-ups, etc. to form a wide area network (“WAN”) or intranet that may span an entire city, country or continent. An organization employing multiple intranets, moreover, may interconnect them through the Internet. Remote users may also utilize the Internet to contact and exchange information with the organization's intranet.
One or more intermediate network devices are often used to couple LANs together and allow the corresponding entities to exchange information. For example, a bridge may be used to provide a “bridging” function between two or more LANs or a switch may be utilized to provide a “switching” function for transferring information between a plurality of LANS. A router is often used to interconnect LANs executing different LAN standards, to interconnect two or more intranets and/or to provide connectivity to the Internet. Routers typically provide higher level functionality than bridges or switches.
In order to reduce design complexity, most computer networks are organized as a series of “layers”. Each layer implements particular rules and conventions referred to as a protocol. The set of layers, moreover, are arranged to form a protocol stack. One of the most widely implemented protocol stacks is the Transmission Control Protocol/Internet Protocol (TCP/IP) Reference Model. The TCP/IP Reference Model defines five layers, which are termed, in ascending order: physical, data link, network, transport and application. The TCP/IP Reference Model basically provides a packet switched or connection-less communication service. That is, each message from a source to a destination carries the full address of the destination, and each one is routed through the network independent of the others. As a result, two messages from the same source to the same destination may follow different routes or paths through the network, depending on the level of congestion and other factors present at the time the two messages are sent.
To interconnect dispersed computer networks, many organizations rely on the infrastructure and facilities of service providers. For example, an organization may lease a number of T1 lines to interconnect various LANs. These organizations typically enter into service level agreements (SLAs) with the service providers, which include one or more traffic specifiers. The traffic specifiers may place limits on the amount of resources that the subscribing organization will consume for a given charge. For example, a user may agree not to send traffic that exceeds a certain bandwidth (e.g., 1 Mb/s). Traffic specifiers may also state the performance characteristics that are to be guaranteed by the service provider. For example, certain applications, such as videoconferencing and audio or voice applications, are highly susceptible to latency in the network. For example, voice over IP applications typically require less than 150 milliseconds of one-way delay time.
As a result, service providers and network managers are interested in determining the latency of their networks. One method to measure latency is to simply time stamp a message, send it across the network and determine how long it takes to reach its destination. However, as described above, two messages sent from the same source to the same destination may follow entirely different paths across a network. Accordingly, this approach may yield different results each time it is performed, since each message may follow a different path. Thus, to be meaningful, latency should refer to a specific path through the network.
The Internet Protocol (IP) provides a mechanism, known as source routing, for ensuring that a message follows a specific, predetermined network path. With source routing, each message carries the list of intermediate devices that the message must visit as it travels from the source to the destination. By specifying a particular set of devices, a message can be constrained to follow a select path. Source routing is implemented by adding a particular option to each message.
FIG. 1A is a block diagram of a conventional network layer message 100 that complies with version 4 of the IP protocol. Message 100 includes an IP header 102 and a data portion 104. The IP header 102 consists of a plurality of fields, including a version field 106, an IP header length field 108, a type_of_service (ToS) field 110, a total message length field 112, an identification field 114, a flags field 116 and a fragment offset field 118. Additional header fields include a time to live (TTL) field 120, a protocol field 122, which specifies the transport layer protocol to which message 100 should be passed, and a checksum field 124. The IP header 102 of each message 100 further includes an IP source address (IP SA) field 126 that identifies the source of the message 100 and an IP destination address (IP DA) field 128 that specifies the intended recipient of the message 100.
If desired, one or more options may be added to the IP header 102 following the IP DA field 128 in an options area 130. For example, options area 130 may include a source routing option 132. The IP protocol actually specifies two options that relate to source routing: strict source routing and loose source routing. In strict source routing, the entire list of layer 3 devices, such as routers and layer 3 switches, through which message 100 must pass as it travels from the source to the destination is specified. In loose source routing, only those layer 3 devices that message 100 must not miss as it travels from the source to the destination are identified. Source routing option 132 similarly includes a plurality of fields, such as a type field 134 (which is set to “131” for loose and “137” for strict routing), a length field 136 that specifies the length of option 132, a pointer field 138 and a route data field 140 that contains the list of layer 3 devices to be visited. Within data field 140, the layer 3 devices are identified by their IP addresses. The value of pointer field 138, moreover, identifies the particular address within route data field 140 to which message 100 is to be forwarded. Thus, before transmitting message 100, a layer 3 device advances the pointer of field 138 to identify the next address in the list.
IP message 100 may be encapsulated in a transport layer message. FIG. 1B is a partial block diagram of a transport layer message 150. The transport layer message 150 preferably includes a source port field 152, a destination port field 154 and a data field 156, among others. Fields 152 and 154 are preferably loaded with the predefined or dynamically agreed-upon port numbers for the particular transport layer protocol (e.g. TCP, the User Datagram Protocol (UDP), etc.) that is being utilized by the respective entities.
To measure the latency of a specific network path, a host could use the IP source routing option described above. In particular, the host could generate an IP message containing a source routing option (preferably strict) that specifies all of the layer 3 devices along the path of the interest. The host would then time stamp and transmit the message. Since the message is constrained to follow the specified path, by virtue of the source routing option, the time it takes for the message to go from the host to the destination is the latency of that path. Unfortunately, there is a substantial drawback of this approach.
Modern layer 3 devices typically include both a routing processor and a switching processor. The routing processor is utilized to perform high level route processing functions on received messages, such as identifying the best path for use in forwarding the messages, and other functions. The routing processor typically stores the received messages in a temporary buffer or register while these functions are being performed. The switching processor, on the other hand, simply moves the received messages from an input router interface to an output router interface based on “shortcuts” derived from earlier decisions rendered by the routing processor. Because the switching processor moves traffic with simpler determinations than those performed by the routing processor, latency within the router or layer 3 device can be significantly reduced by moving traffic with just the switching processor.
To the extent network layer messages include one or more options, the messages must be evaluated by the routing processor since switching processors are generally not configured to process options. By examining the value of header length field 108, a switching processor can quickly determine whether or not a given message includes any options. If the length of an IP header 102, as reflected in header length field 108, is greater than 5 octets, the switching processor “knows” that it contains at least one option. In response, the switching processor passes the message to the routing processor for further processing. At the routing processor, the message is placed in a temporary buffer or register while the routing processor determines which options are included in the message and performs the specified functions. As a result, messages containing options typically suffer a higher latency than messages that carry no options.
This added latency for messages carrying source routing options renders the corresponding latency determinations inaccurate. That is, the latency experienced by a message having a source routing option is often greater than a message carrying no options, since the source routing option must be evaluated by the routing processor of each layer 3 device at which the message is received. Since most messages, including those generated by video and audio applications, do not include options, basing latency determinations on messages carrying source routing options leads to inaccurate results. Accordingly, a need exists for a mechanism to measure the latency of selected network path with greater accuracy.
It is an object of the present invention to provide a system and method for accurately measuring the latency of a selected path in a computer network.